Added Spells Compendium with icons

I've added every rulebook spells into a compendium with an icon from
game-icons.net. The compendium isn't functional, but has every rule in
it.

src/system.json

@@ -52,6 +52,14 @@
             "module": "ds4",
             "path": "./packs/items.db",
             "entity": "Item"
+        },
+        {
+            "name": "magic-spells",
+            "label": "Zauber (GRW)",
+            "system": "ds4",
+            "module": "ds4",
+            "path": "./packs/magic-spells.db",
+            "entity": "Item"
         }
     ],
     "languages": [